Optical isolators and circulators are generally known to those skilled in the art. Generally speaking an optical circulator takes light in at a first port and outputs the light at a second port. However, light coming in the second port does not come out the first port but rather comes out a third port. (In an isolator there are only two ports, while in a circulator there are more than two ports). Thus an optical circulator is said to be a "non-reciprocal" device based on the above behavior. U.S. Pat. No. 5,204,771 to Koga, incorporated by reference herein, generally discloses various types of optical circulators. For example, FIG. 4 of that patent discloses an optical circulator 21 comprised of walk-off double refraction crystal plates 22, 23, and 24 and rotators 25 and 26. Light coming in port 27 exits out port 29. Light coming in port 29 exits out port 28.
U.S. Pat. No. 5,471,340 to Cheng et. al., incorporated by reference herein, also discloses various optical circulators. For example, FIG. 1A of that patent discloses walk-off birefringement crystals 24 and 32, half-wave plates 26 and 28, Faraday rotators 30 and 34, and mirror 36. Light coming in port 1 exits port 2, while light coming in port 2 exits port 3.